The invention relates to an energy storage apparatus for an onboard power supply system of a motor vehicle. The energy storage apparatus includes a first electrical energy store, which is characterized by a first voltage characteristic that stipulates the quiescent voltage of the first electrical energy store on the basis of its relative state of charge, and a second electrical energy store, which is connectable in parallel with the first electrical energy store via a controllable switching element and is characterized by a second voltage characteristic that stipulates the quiescent voltage of the second electrical energy store on the basis of its relative state of charge. A first voltage value range, covered by the first voltage characteristic, and a second voltage value range, covered by the second voltage characteristic, partially overlap. Further, the energy storage apparatus includes a device for determining the state of charge of the first energy store and/or of the second energy store.
An onboard power supply system of a motor vehicle is used particularly for providing electric power for starting the motor vehicle and for operating electrical loads. In order to increase the certainty of the motor vehicle starting, onboard power supply systems having two rechargeable energy stores connected in parallel are known. A first energy store is usually a lead battery in the form of multiple lead-based storage elements, also referred to as cells. A second energy store, connected in parallel therewith, may be in the form of a powerful lithium ion battery, for example. In combination, functionalities such as automatic stopping and starting of the engine during vehicle operation and efficient braking energy recovery when the vehicle is coasting can be implemented. The provision of the second energy store allows the availability of load functions to be increased and the life of the first energy store to be extended. A vehicle onboard power supply system having two energy stores connected in parallel is also referred to as a dual energy store onboard power supply system.
Such a dual energy store onboard power supply system for a vehicle is known from DE 10 2010 062 116 A1, for example. In the case of this onboard power supply system, the two energy stores, whose voltage characteristics partially overlap, are connected in parallel in voltage-neutral fashion via a switch, a relay and/or hard wiring.
DE 10 2006 048 872 A1 proposes connecting a relay between a lead battery in the form of a starter battery and a power supply battery in the form of a lithium ion phosphate battery, the relay being actuable directly by an ignition lock of the motor vehicle. The starter battery is connected to the onboard power supply system of the motor vehicle exclusively when the ignition is switched on, the relay being closed in this case. The power supply battery is connected to the onboard power supply system of the motor vehicle both when the ignition is switched on and when the ignition is switched off, which means that electrical loads can be supplied with voltage or electric current by the power supply battery even when the ignition is switched off.
In dual energy store onboard power supply systems of this kind, attempts are made to connect the two energy stores in parallel for the longest possible proportions of time, this being ensured in most cases by the substantially longer idle time in proportion to travel time. As a result, the system voltage appearing in the onboard power supply system is a mixed potential. The mixed potential does not necessarily correspond to the single voltage of the energy stores, if these were electrically isolated from one another. This makes it difficult to determine the state of charge of the two energy stores, which, in the case of onboard power supply systems having only one energy store, is determined from a relationship between the quiescent voltage of the energy store and the state of charge.
It is an object of the present invention to provide an energy storage apparatus for a dual energy store onboard power supply system of a motor vehicle in which the state of charge can be determined in a simple and reliable manner. It is a further object of the invention to provide a method for operating such an energy storage apparatus that allows the state of charge of the energy stores to be determined in a simple and reliable manner.
These objects are achieved by an energy storage apparatus and a method for operating the energy storage apparatus in accordance with embodiments of the invention.
According to a first aspect, an energy storage apparatus for an onboard power supply system of a motor vehicle includes a first electrical energy store, a second energy store and a device for determining the state of charge of the first energy store and/or of the second energy store. The first electrical energy store is characterized by a first voltage characteristic that stipulates the quiescent voltage of the first electrical energy store on the basis of its relative state of charge. The second electrical energy store is connectable in parallel with the first electrical energy store via a controllable switching element. The switching element may be an internal switching element of the second energy store or an external switching element that is interconnected in series with the second energy store. The switching element may be provided, by way of example, for undertaking safety functions, in particular such as overvoltage protection, undervoltage protection or temperature protection, of the second energy store. The second electrical energy store is characterized by a second voltage characteristic that stipulates the quiescent voltage of the second electrical energy store on the basis of its relative state of charge. In this case, a first voltage value range, covered by the first voltage characteristic, and a second voltage value range, covered by the second voltage characteristic, partially overlap. The device for determining the state of charge of the first energy store and/or of the second energy store is designed to determine the state of charge either by way of a quiescent voltage measuring device for determining the state of charge on the basis of a quiescent voltage and/or by way of a state of charge determination device for determining the state of charge by balancing a respective battery current. The device for determining the state of charge is designed to open the controllable switching element when the motor vehicle is in a quiescent-state-like state and, at the same time, a quality value, which was determined during determination of the state of charge at an earlier time and in parallel with said state of charge and which expresses the quality of the balancing, exceeds a predefinable threshold value. The device for determining the state of charge is further designed to activate the quiescent voltage measuring device for determining the state of charge of at least the first energy store when these two criteria are met.
The energy storage apparatus allows determination of the state of charge by way of quiescent voltage measurement that is performable relatively accurately, such as that known from conventional battery systems. In this case, the state of charge is determined on the basis of the direct relationship between the state of charge and the quiescent voltage of the energy store to be measured. Such quiescent voltage measurement is performed, as in the case of known energy stores, as soon as a defined time in which there is no or almost no flow of current to the energy store or from the energy store has been identified. Between two quiescent voltage measurements, the state of charge is continued by balancing the battery current. Since the tolerance of the balanced state of charge becomes greater as the converted amount of charge increases, allowance is made for this circumstance by performing a quiescent state measurement only when the tolerance of the state of charge determined by balancing is too great (i.e. there is excessive uncertainty about the actual state of charge), after the vehicle for which there are quiescent-state-like conditions is switched off.
As a result, it is therefore possible to determine a sufficiently accurate state of charge of at least the first energy store, wherein there is the least possible restriction of the envisaged coupled operation. This is because the coupling between the two energy stores can be maintained whenever the quality value expressing the tolerance of the state of charge is good (i.e. the tolerance is low), which means that coupled operation of the two energy stores continues to be possible even when the vehicle is switched off. Furthermore, the energy storage apparatus can be implemented without additional hardware. It is possible to use known algorithms for a respective technology of the energy stores, which allows inexpensive implementation of the energy storage apparatus.
The device for determining the state of charge can, according to a further configuration, be designed to increment or decrement the quality value when the quality of the balancing by means of determination of the state of charge by the state of charge determination device decreases, and to decrement or increment the quality value in the opposite direction when the state of charge is determined by the quiescent voltage measuring device. The incrementation or alternatively decrementation of the quality value is effected as part of the determination of the state of charge by the state of charge determination device as the converted quantity of charge increases. The quality value allows for the circumstance that when the current is integrated with respect to time, certain errors can arise that become more and more noticeable as the length of time increases and result in a greater tolerance in the state of charge determined by computation. By contrast, determination of the state of charge by way of a quiescent voltage measurement using known algorithms is reliably performable, that the quality value can then be adapted as appropriate in the opposite direction.
The device for determining the state of charge may further be designed to open the controllable switching element with a time delay after the quiescent-state-like state is found, i.e. when or after the vehicle has changed to the quiescent-state-like state. In other words, this means that the switching element is not opened until after a defined parking time. This allows frequent operation of the switching element to be avoided during short-range operation of the vehicle.
The device for determining the state of charge may further be designed to repeat the determination of the state of charge by way of the quiescent voltage measuring device at prescribed intervals of time when or after the vehicle has changed to the quiescent-state-like state. This allows the quality the determination of the state of charge to be performed with greater accuracy as the parking time increases. As intervals of time, it is possible for measurements to be performed every hour, every half hour or the like, for example.
The device for determining the state of charge may further be designed to determine, i.e. to adapt, the quality value whenever the state of charge is determined by the quiescent voltage measurement. This ensures that—when considered over time—not every parking maneuver for which there is the quiescent-state-like state prompts the switching element to be opened and the state of charge to be determined by a high state measurement. Instead, the switching element can remain closed in many cases of a switched-off vehicle, which allows the first energy store to be charged by the second energy store, for example.
According to a further configuration, the device for determining the state of charge may be designed to close a controllable switching element when or as soon as the quiescent-state-like state of the motor vehicle is left. This ensures that both energy stores are already linked to the vehicle onboard power supply system for the starting procedure in order to allow for the high currents needed for the starting process.
The device for determining the state of charge may be a superordinate controller of the first and the second energy store. The superordinate controller may be connected to a respective appropriate control system, e.g. a battery sensor, of the first and the second energy store for the purpose of interchanging data. Alternatively, the device for determining the state of charge may be arranged in the first and second energy stores in a distributed manner. In this case, the first and second energy stores have, besides appropriate measuring and processing devices, means for direct communication. In this second variant, the superordinate controller can perform merely the rating.
According to one configuration, the first energy store is a lead acid storage battery. By contrast, the second energy store is a lithium ion storage battery. Such an energy storage apparatus, the operation thereof and advantages for the supply of power to a vehicle onboard power supply system are described by way of example in DE 10 2010 062 116 A1 from the applicant.
According to a second aspect of the present invention, a method for operating an energy storage apparatus for an onboard power supply system of a motor vehicle is provided. The energy storage apparatus includes a first electrical energy store, a second electrical energy store and a device for determining the state of charge of the first energy store and/or of the second energy store. The first electrical energy store is characterized by a first voltage characteristic that stipulates the quiescent voltage of the first electrical energy store on the basis of its relative state of charge. The second electrical energy store is connectable in parallel with the first electrical energy store via controllable switching element. It is characterized by a second voltage characteristic that stipulates the quiescent voltage of the second electrical energy store on the basis of its relative state of charge. In this case, a first voltage value range, covered by the first voltage characteristic, and a second voltage value range, covered by the second voltage characteristic, partially overlap. The device for determining the state of charge of the first energy store and/or of the second energy store is designed to determine the state of charge either by a quiescent voltage measuring device for determining the state of charge on the basis of a quiescent voltage or by a state of charge determination device for determining the state of charge by balancing a respective battery current.
The method involves the following steps being performed: determination of the state of charge of the first energy store and/or of the second energy store by the state of charge determination device via balancing of respective battery currents; parallel (i.e. simultaneous) determination of a quality value that expresses the quality of the balancing; opening of the controllable switching element when, as a first criterion, the motor vehicle is in a quiescent-state-like state and, as a second criterion, the quality value exceeds a predefined threshold value, and activation of a quiescent voltage measuring device for determining the state of charge of at least the first energy store in a quiescent-state-like state of the vehicle.
The method according to the invention has the same advantages as have been described above in connection with the energy storage apparatus according to the invention.
According to an expedient configuration of the method according to the invention, the quality value is incremented or decremented when the quality of the balancing by way of determination of the state of charge by the state of charge determination device decreases, and the quality value is decremented or incremented in the opposite direction when the state of charge is determined by the quiescent voltage measuring device.
According to a further configuration of the method, the controllable switching element is opened with a time delay after the quiescent-state-like state is found.
According to a further expedient configuration of the method, the determination of the state of charge by the voltage measuring device is repeated at prescribed intervals of time.
According to a further expedient configuration, the quality value is determined or adapted whenever the state of charge is determined by the quiescent voltage measuring device. In other words, the quality value is decremented or incremented in the aforementioned opposite direction.
According to a further expedient configuration of the method, the controllable switching element is closed when or as soon as the quiescent-state-like state of the motor vehicle is left.
The method according to the invention can be performed by a superordinate controller of the first and the second energy store, determination of the state of charge being performed by a respective unit, e.g. a battery sensor, of the first and second energy stores. In particular, the superordinate controller then delivers the signal to open or close the switching element, according to need.
In an alternative configuration, the method can also be performed by a unit, e.g. a battery sensor, of at least one of the energy stores. In particular, one of the two energy stores then delivers an appropriate signal to actuate the switching element to perform the above-described quiescent state measurement when the vehicle is switched off.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.